In wireless communication systems, inevitable influence of terrains or obstacles on signals causes the occurrence of multipath distortion. As a time-varying channel impulse response is generally modeled as a time-domain discrete finite impulse response (FIR) filter denoted by
            h      ⁡              (                  τ          ;          t                )              =                  ∑        n            ⁢                                    a            n                    ⁡                      (            t            )                          ⁢                  ⅇ                                    -              j                        ⁢                                                  ⁢            2            ⁢            π            ⁢                                                  ⁢                          f              c                        ⁢                                          τ                n                            ⁡                              (                t                )                                                    ⁢                  δ          ⁡                      (                          τ              -                                                τ                  n                                ⁡                                  (                  t                  )                                                      )                                ,complex multipath interference always exists in received wideband signals, which appears as frequency selective fading in the frequency domain. For this reason, single-carrier systems usually employ time domain equalization to eliminate the multipath influence, which results in a very high complexity of receivers.
The orthogonal frequency division multiplexing (OFDM) technology has been widely used in wireless wideband communication systems. One significant advantage of OFDM technology is its capability of dividing a carrier with a relatively wide bandwidth into multiple parallel subcarriers, each subcarrier having a bandwidth far less than the coherence bandwidth of a channel. Therefore, the channel frequency fading that each subcarrier signal undergoes is flat, which overcomes the adverse effect of channel frequency selective fading. If channel frequency response characteristics at different subcarriers can be obtained by channel estimation technology, a receiver will be capable of realizing coherent demodulation to correctly recover transmitted signals. In order to improve demodulation threshold of the receiver and the quality of received signals, noise reduction is generally performed to the estimated channel response.
Wiener filtering is a commonly used channel noise reduction technology in multi-carrier OFDM systems. In consideration of the implementation complexity, Wiener filtering methods are usually designed according to several groups of preset channel power delay characteristics, and the corresponding Wiener filtering coefficients are stored. The receiver selects appropriate Wiener filtering methods and coefficients according to the preset channel power delay characteristic in the practical transmission environment. The above concept is applied in the China mobile multimedia broadcasting (CMMB) system to perform noise reduction and interpolation by selecting the most appropriate group of Wiener filtering coefficients through analyzing the multipath delay spread in a transmission channel. However, this method is only suitable for CMMB systems or the like, which adopt a low quadrature amplitude modulated (QAM) transmission mode, as the low QAM transmission mode is not sensitive to weak paths in reception environment, and ignorance or loss of several weak paths will not effect the determination of constellation points. Moreover, as the energy of noise in the environment where a low QAM reception is located is relatively high, it is advantageous for the reception threshold if some weak paths together with the noise are inhibited. With the increase of transmission rate, great QAM constellation points are more frequently used in practical systems, for example, the 64-QAM constellation points used in wireless LANs and the 256-QAM constellation points used in the European DVB-T2 system. These constellation points all have high demodulation thresholds, so that weak multipath in the transmission environment will have significant effect on their determination. Nevertheless, virtual carriers employed in OFDM systems that include cyclic prefix make weak paths of low energies sometimes be drowned in energy leakage of strong paths and cannot be easily detected, making the channel noise reduction of high QAM reception systems more difficult. In fact, the channel noise reduction process will lead to channel distortion while reducing the noise. As noise reduction is good for demodulation and channel distortion is bad for demodulation, the result of the channel noise reduction process is depended on the combined effect of the above two opposing effects.
FIG. 1 illustrates the frame structure of a multi-carrier transmission mode in 802.11a/g systems. The frame consists of four parts wherein a first part is a short training sequence; a second part is a long training sequence; a third part is a signaling sequence; and a fourth part is a data sequence. The short training sequence is composed of 10 duplicate short training symbols and is mainly used for signal gain adjustment, signal capture and coarse estimation of carrier frequency offset. The following long training sequence is composed of 2.5 duplicate long training symbols and is mainly used for accurate estimation of carrier frequency offset, accurate timing synchronization of OFDM symbols and multipath estimation of transmission channel. The following signaling sequence is used for transmitting indication information which is necessary for the demodulation of the data sequence. Such indication information may include the length of the data sequence as well as the modulation mode used in transmission and the coding efficiency. Once the receiver obtains the above mentioned necessary information, it will be able to correctly demodulate and detect the data sequence.
In the 802.11a/g multi-carrier mode, channel estimation is accomplished during the period of receiving the long training sequence. The frequency domain channel response is obtained by dividing the received long training sequence by the locally stored frequency domain long training sequence. Since the frequency domain channel contains noise in most cases, if the channel is directly used for subsequent demodulation operation, it will result in a poor demodulation performance, therefore a noise reduction process is usually performed to the channel to improve operation threshold of system.